This invention relates to a planetary gear transmission which provides a high change gear ratio, though its structure is compact.
As is known in the art, a transmission comprising a planetary gear mechanism can provide a greater change gear ratio while its structure is more compact than ordinary change gear transmissions. The greater the gear ratio of a planetary gear to a sun gear, that is, the greater the ratio of the diameter of the planetary gear to that of the sun gear, the greater becomes this change gear ratio.
However, since the planetary gear mechanism has a plurality of planetary gears disposed around the sun gear, the change gear ratio obtained from one particular planetary gear mechanism is limited to the extent such that the diameters of the plurality of planetary gears do not interfere with one another. Two or more sets of planetary gear mechanisms must be coupled to each other in an axial direction in order to obtain a greater change gear ratio. If two or more sets of planetary gear mechanisms are coupled to each other in an axial direction, however, the size of the overall device in the axial direction becomes large, and a larger installation space becomes necessary. For this reason, the cost of production of the transmission becomes unavoidably higher.